Method of alloying prosphorus and nickel in a fused salt bath



United States Patent 3,236,752 METHOD OF ALLOYING PROSPHORUS AND NICKEL IN A FUSED SALT BATH Dwight E. Couch, Boulder City, Nev., assignor to the United States of America as represented by the Secretary of Commerce No Drawing. Filed Apr. 4, 1963, Ser. No. 270,772 5 Claims. (Cl. 204-39) This invention relates to a method of alloying phosphorus and nickel, and more particularly relates to a method of forming a coating of nickel-phosphorus alloy on articles having nickel surfaces.

There are various known methods of forming hard, protective coatings of nickel-phosphorus alloy on articles, in which nickel and phosphorus are co-deposited to form the alloy. Since many articles are fabricated from nickel, or are normally nickel-plated during the manufacture thereof, it would be desirable to provide a method of diffusing phosphorus into the nickel surfaces thereof, to provide the desired coating of nickel-phosphorus alloy.

Accordingly, an object of this invention is to provide a simple, economical method of forming a hard, corrosionresistant coating of nickel-phosphorus alloy on a nickelsurfaced article. Another object is to provide a method of forming nickel-phosphorus alloy coatings of any desired thickness on nickel-surfaced articles.

These and other objects are attained in accordance with this invention by a process in which phosphorus is deposited out of a molten salt bath onto a nickel-surfaced article.

Other objects, advantages and features of the present invention will become more apparent from the following description of a preferred embodiment thereof.

In accordance with this invention, the article or piece to be coated with nickel-phosphorus alloy is either fabricated from nickel, or is nickel-plated by any suitable process, so as to provide a nickel surface into which phosphorus may be diffused.

Further, in accordance with this invention, a fused salt bath is prepared by melting a mixture of an alkali metal metaphosphate and one or more alkali metal chlorides in a container of suitable size and shape. The alkali metal metaphosphate provides the required elemental phosphorus, and should comprise at least about 2% by weight of the total mixture. Preferably, it comprises about 7% to about 100% by weight of the total mixture, to insure that the phosphorus concentration is sufficient to achieve the desired diffusion within a con venient length of time. The alkali metal chlorides serve as diluents capable of lowering the melting point of the mixture. They may be present up to about 98% by weight of the total mixture; for the reason discussed above, it is preferred that they comprise about 93% or less of the mixture.

The fused salt bath is maintained at a temperature within the range of about 400 C. to about 1000" C. When moderate amounts of the alkali metal chloride diluents are employed, the temperature may be maintained between about 400 C. and about 700 C., thus effecting a savings in the heat required for operation of the bath.

The nickel-surfaced article to be coated is immersed in the fused salt bath. If a thin coating less than about 0.0005 inch of nickel-phosphorus alloy is desired, the article is kept immersed in the bath for a short time and then removed. The time is not critical and may range from about 5 seconds to about 5 minutes. After removal from the bath, the article is cooled in air to about room temperature, washed in flowing water to remove any salts adhering thereto, and dried.

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If thicker coatings of nickel-phosphorus alloy are desired to be formed on the nickel-surfaced article, the fused salt bath is electrolyzed, with the immersed article connected as the cathode. The current density is adjusted within the range of about one ampere to about 200 amperes per square decimeter of surface of the article. In general, a current density of about 10 amperes to about amperes per square decimeter is preferred, since densities below about 10 amperes per square decimeter require inconveniently long plating times, while densities above about 100 amperes per square decimeter result in coatings that are porous and dark in color. In most instances, an operating density of about 25 amperes to about 50 amperes per square decimeter has proved to be satisfactory, resulting in the formation of about 0.007 inch to about 0.010 inch of alloy coating per hour of electrolysis. After the desired thickness of coating has been formed, the article is removed from the bath and cooled, washed, and dried as described above.

The following examples illustrate the invention.

Example I About 10 grams of sodium metaphosphate, 74 grams of potassium chloride, and 38 grams of sodium chloride were melted together in a graphite crucible, and maintained at a temperature of about 600 C. to about 900 C. A nickel-plated sheet was immersed in the bath for about one minute, removed, cooled in air to about room temperature, washed in water, and dried. The coating had a hardness of about 580 Knoop.

Example II About 10 grams of potassium metaphosphate, 74 grams of potassium chloride, and 42 grams of lithium chloride were melted together in a graphite crucible, and maintained at a temperature of about 400 C. to about 700 C. A nickel-plated sheet was immersed in the bath and connected as the cathode, the graphite crucible being the anode. A direct current of about 25 amperes to about 50 amperes per square decimeter of surface of the immersed sheet was passed through the bath for one hour, after which the sheet was removed, air-cooled, washed in water, and dried. The coating had a hardness of about 580 Knoop.

Example 111 About grams of sodium metaphosphate was melted in a graphite crucible and maintained at a temperature of about 800 C. to about 1000 C. Using a nickel sheet as cathode, the bath was electrolyzed at a current density of about 25 amperes to about 50 amperes per square decimeter of surface of the sheet for about one hour. The sheet was then removed, air-cooled, to about room temperature, washed under flowing water, and dried. The coating hardness tested to about 580 Knoop.

From the foregoing it will be appreciated that the present invention provides a simple, economical method of forming hard, protective coatings of nickel-phosphorus alloy on nickel or nickel-plated articles. The method enables one to deposit coatings of any desired thickness in a controllable manner.

Since various modifications of the process as described can be made within the scope of the invention, it is intended that the invention not be limited to the details of the specific processes disclosed, except as defined by the appended claims.

What is claimed is:

1. The method of forming a coating of nickelphosphorus alloy on an article having a nickel surface which comprises treating said article in a fused salt bath consisting essentially of an alkali metal metaphosphate and up to about 98% by weight of alkali metal chlorides.

2. The method defined in claim 1, wherein the temperature of said fused salt bath is in the range of about 400 C. to about 1000 C.

3. The method of forming a coating of nickelphosphorus alloy on an article having a nickel surface which comprises electrolyzing a fused salt bath consisting essentially of an alkali metal metaphosphate and up to about 98% by weight of alkali metal chlorides in an electrolytic cell in which said article is cathodic.

4. The method defined in claim 3, wherein the temperature of said fused salt bath is in the range of about 400 C. to about 1000 C.

References Cited by the Examiner UNITED STATES PATENTS 7/1935 McCauley l48-6.1l 1/1942 MacKay 1486.11

WINSTON A. DOUGLAS, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

1. THE METHOD OF FORMING A COATING OF NICKELPHOSPHORUS ALLOY ON AN ARTICLE HAVING A NICKEL SURFACE WHICH COMPRISES TREATING SAID ARTICLE IN A FUSED SALT BATH WHICH COMPRISES TREATING SAID ARTICLE IN A FUSED SALT BATH CONSISTING ESSENTIALLY OF AN ALKALI METAL METAPHOSPHATE AND UP TO ABOUT 98% BY WEIGHT OF ALKALI METAL CHLORIDES.
 3. THE METHOD OF FORMING A COATING OF NICKELPHOSPHORUS ALLOY ON AN ARTICLE HAVING A NICKEL SURFACE WHICH COMPRISES ELECTROLYZING A FUSED SALT BATH CONSISTING ESSENTIALLY OF AN ALKALI METAL METAPHOSPHATE AND UP TO ABOUT 98% BY WEIGHT OF ALKALI METAL CHLORIDES IN AN ELECTROLYTIC CELL IN WHICH SAID ARTICLE IS CATHODIC. 